The proposed hydrogen economy has great promise as a clean alternative to fossil fuels. Hydrogen gas is first generated from a primary energy source (such as a nuclear plant or a renewable energy source) and is then stored. The stored H2 is transported to a point-of-use, where energy is generated either by direct combustion of hydrogen gas or by using it as a feed component in an electrochemical fuel cell.
In principle, the combination of a fuel cell and a hydrogen storage device provides an environmentally friendly power source capable of application in any situation where electricity is required. Such applications include on-site uses such as industrial facilities (for example, those at inaccessible locations such as oil rigs), laboratories, back-up systems for key buildings (for example, hospitals), traffic lights and remote sensors. On-board applications include vehicles, laptop computers, mobile telephones and personal music players.
A number of practical difficulties present a significant obstacle to the immediate, widespread use of hydrogen as an environmentally friendly energy source. A suitable hydrogen storage device must fulfil a number of criteria, including: high storage capacity (by weight); high storage density; favourable kinetics and temperatures/pressures for release of hydrogen; and safe containment of H2 prior to its release. For many applications, for example on-board applications, the storage of hydrogen in pressurised or cryogenically cooled form is impractical for a number of reasons. These include the still relatively poor storage density of pressurised or liquid H2, the weight contribution of the apparatus required to achieve the necessary pressurisation/liquefaction, and (in the case of liquid H2) the high energy cost of maintaining cryogenic cooling.
Hydrogen storage densities higher than that of liquid hydrogen can be achieved by storing the H2 in a storage material. Alkali metal borohydrides have been extensively studied as potential hydrogen storage materials. These compounds can be caused to release hydrogen either by thermal decomposition or hydrolysis. Lithium borohydride (LiBH4), sodium borohydride (NaBH4) and potassium borohydride (KBH4) have hydrogen contents of 18.5%, 10.7% and 7.5% by weight, respectively. These values compare favourably with, for example, the US Department of Energy's 2010 mobile storage target of 6.5 wt %.
Sodium borohydride is the only alkali metal borohydride to have been marketed as a hydrogen storage material on a commercial level until now. However, the release of hydrogen from this material by hydrolysis requires the use of a ruthenium catalyst, while the release of hydrogen by thermal decomposition requires heating to 400° C.